1. Field of the Invention
The present invention relates to a scanning electron microscope for obtaining a scanned image of a specimen by scanning an electron beam over the specimen.
2. Description of Related Art
The structure of a general scanning electron microscope is shown in FIGS. 1A and 1B. FIG. 1A shows the lens arrangement of the scanning electron microscope. FIG. 1B shows the orbit of an electron beam through the lens arrangement. In the electron beam orbit, the angle is exaggerated about 10 times for ease of understanding.
An electron gun 50 that is an electron source has an emitter 1, an extraction electrode 2, and acceleration electrodes 3. An extraction voltage is applied to the extraction electrode 2. As a result, electrons are extracted from the emitter 1 and accelerated by an acceleration voltage applied to the acceleration electrodes 3. Then, the electrons are released as an electron beam 1b toward a specimen 14.
A first condenser lens 11, a second condenser lens 101, and an objective lens 13 are arranged between the electron gun 50 and the specimen 14. The second condenser lens 101 may also be referred to as an angular aperture control lens.
An objective aperture plate 12 is disposed close to the specimen side of the first condenser lens 11. An electron detector 102 is disposed close to the specimen side of the second condenser lens 101. The objective aperture plate 12 and electron detector 102 are provided with apertures 12a and 102a, respectively, through which the electron beam passes. These apertures 12a and 102a are located on the optical axis 1a of the electron optical system.
The first condenser lens 11 acts to vary the current value (probe current value) of the electron beam impinging on the specimen 14. The second condenser lens 101 adjusts the orbit of the electron beam such that the electron optical system is optimized for the aberration in the objective lens 13. In FIG. 1B, the first condenser lens 11 is operated in a real imaging mode, while the second condenser lens 101 is operated in a virtual imaging mode as one example.
Where the probe current value of the electron beam is varied using such a lens system, the excitation of the first condenser lens 11 is varied (e.g., the excitation is weakened) or the selected aperture 12a of the objective aperture plate 12 is manually switched to another aperture of a different diameter. At this time, the state of excitation of the second condenser lens 101 is also varied so as to minimize the probe diameter of the electron beam over the specimen 14.
Another lens arrangement as shown in FIGS. 2A and 2B is also available. FIG. 2A shows the lens arrangement of the scanning electron microscope. FIG. 2B shows the orbit of the electron beam through the lens arrangement. In FIGS. 1A, 1B, 2A, and 2B, like components are indicated by like reference numerals.
In the structure shown in FIGS. 2A and 2B, an objective aperture plate 113 is provided with plural apertures 113a of different diameters. Deflectors 111 and 112 are disposed above and below, respectively, the objective aperture plate 113 to permit the selected aperture 113a of the objective aperture plate 113 through which the electron beam 1b passes is switched among the apertures in the objective aperture plate 113. In this case, the deflectors 111, 112 and objective aperture plate 113 are preferably disposed close to the electron gun 50.
In any example of the above-described lens arrangements, the electron detector 102 is disposed in a position located beside the optical axis 1a of the electron optical system. Therefore, electrons released from the specimen 14 by electron beam irradiation undergo the lens action of the electron optical system until the electrons are detected by the detector 102. Consequently, in designing the electron optical system, behavior of the electrons to be detected needs to be discussed at the same time.
Electron detectors disposed along the optical axis 1a of the electron optical system in this way are often used to detect elastically scattered electrons having small emission angles (about 5 to 30°) with respect to a reference angle (0°) that is the optical axis 1a of the electron optical system. Electrons released from the specimen 14 are affected to different extents by spherical aberration, depending on their different emission angles.
Spherical aberration more strongly focuses electrons having larger emission angles out of electrons emitted from the specimen 14. Therefore, elastically scattered electrons having smaller emission angles are less affected by spherical aberration and assume orbits similar to the orbit of the electron beam (primary electron beam) directed at the specimen 14. For this reason, in order to detect elastically scattered electrons having smaller emission angles, an electron detector needs to be disposed in a position where the primary electron beam is spread.
Accordingly, in the configuration of FIGS. 1A and 1B, the electron detector 102 can efficiently detect scattering electrons emitted at small angles by placing the detector closer to the objective lens 13. The same principle applies to the configuration of FIGS. 2A and 2B. Elastically scattered electrons emitted at smaller angles can be detected by reducing the diameter of the aperture 102a in the electron detector 102.
A well-known technique for detecting elastically scattered electrons is set forth, for example, in U.S. Pat. No. 7,425,701. Note that this well-known technique is a method of detecting and discriminating electrons having small energies from electrons having large energies and that there is no mention of detection of the angles of elastically scattered electrons.
JP-A-2008-153158 sets forth a method of sorting electrons to be detected by making use of an electron detector and the action of a lens disposed behind the detector.
In the example of FIGS. 1A and 1B, a mechanism for manually driving or adjusting the objective aperture plate 12 is necessary to switch the selected aperture 12a of the objective aperture plate 12 to other aperture of a different diameter. Furthermore, the operator of this instrument must adjust the axis of the aperture 12a manually whenever the objective aperture plate 12 is moved and the selected aperture 12a is switched.
In contrast, in the example of FIGS. 2A and 2B, switching of the selected aperture 113a of the objective aperture plate 113 through which the electron beam passes and adjustment of the axis are made by the deflecting action of the same deflection system. Therefore, this technique is free from cumbersome manual operations needed in the technique shown in FIGS. 1A and 1B.
However, the technique shown in FIGS. 2A and 2B has the problem that there are restrictions on the arrangement of the objective aperture plate 113 and deflectors 111, 112.
In particular, the objective aperture plate 113 is provided with plural apertures 113a. When the objective aperture plate 113 is placed remotely from the electron source, the electron beam reaching the objective aperture plate 113 is spread. As a result, a larger ratio of the electron beam passes through the nonelected ones of the apertures 113. Therefore, the objective aperture plate 113 and deflectors 111, 112 need to be placed as close as possible to the electron source.
In the configuration shown in FIGS. 2A and 2B, however, the first condenser lens 11 is located above the deflector 111 and so it is impossible to place the objective aperture plate 113 and deflectors 111, 112 closer to the electron source.
Furthermore, in the configuration shown in FIGS. 2A and 2B, the first condenser lens 11 cannot be operated in the real imaging mode. That is, in this configuration, if the first condenser lens 11 is strongly excited, interference occurs between the focusing field produced by the lens 11 and the deflecting field produced by the upper deflector 111. The focusing field gives a rotating action to the deflecting field. This makes it difficult to control the electron beam.
Consequently, the operation of the first condenser lens 11 is restricted to the virtual imaging mode. Thus, if the probe current of the electron beam is increased, the angular aperture of the objective lens 13 decreases. Since the angular aperture used to enhance the resolution is increased with increasing the probe current, it may be conceivable to mount the second condenser lens operating in real imaging mode behind the lower deflector 112 in accordance with the first condenser lens 11 operating in virtual imaging mode.
However, in the same way as for the first condenser lens 11, there is the problem that interference occurs between the focusing field produced by the second condenser lens and the deflecting field produced by the lower deflector 112. Therefore, a space must be secured to place the second condenser lens. Especially, in this case, the second condenser lens must be excited more strongly than the first condenser lens 11. Hence, the length of the instrument must be increased by an amount corresponding to the space for the second condenser lens.
If the electron detector is placed too close to the specimen, a separate general-purpose detector for detecting elastically scattered electrons and inelastically scattered electrons which are emitted at large angles cannot be placed. Therefore, little latitude is allowed in placing the electron detector. Elastically scattered electrons having small emission angles have small solid angles and contribute only a little to the overall signal. Therefore, if an image arising from electrons detected by the general-purpose detector can be derived at the same time, then increased convenience in use is provided with desirable results.
With respect to the diameters of apertures in the electron detectors, the electron beam is spread from tens of micrometers to hundreds of micrometers at the position of the electron detector. In contrast, the diameters of the apertures formed in the electron detector are on the order of micrometers in practice for the following reason. Assuming that the electron beam deviates from the optical axis 1a of the electron optical system by the effects of lens assembly error and external disturbances, the diameters of the apertures in the electron detectors are increased to grant some latitude.
The lower deflector 112 has a function of adjusting the axis of the electron beam relative to the objective lens 13 but cannot be used for adjusting the axial deviation of the electron beam from the electron detector 102 (aperture 102a). If an axial adjusting mechanism dedicated for the electron detector is added, it is cumbersome to adjust the instrument.